Conventional single-phase rectifier power electronic circuits suffer from high total harmonic distortion (THD) and poor power factor. A number of regulations have been enacted recently to control the harmonic content of line current drawn by the electronic equipment. As a result, researchers have been actively seeking development of power supplies, which can comply with those regulations. In recent years, many circuits and control methods were reported, in which high-frequency switching techniques were used to shape the input current waveform which becomes dominate in power factor correction (PFC). See for example: A. Prasada. P. D. Ziogas and S. Manias. xe2x80x9cA Novel Passing Waveshaping Method for Single-Phase Diode Rectifiers.xe2x80x9d PESC""89. pp. 99-105; L Barbi and S. A. Oliveira da Silva. xe2x80x9cSinusoidal Line Current Rectification at Unity Power Factor with Boost Quasi-resonant Converters.xe2x80x9d In Proceedings of IEEE-APE""90, pp. 553-562; and, P. Kornetzky, H. Wei and L Batarseh. xe2x80x9cA Novel One-Stage Power Factor Correction Converter,xe2x80x9d IEEE APEC""97 Proc., pp. 251-258.
The implementation of high frequency techniques can be classified into two categories, ie. two-stage scheme and one-stage scheme. In a two-stage scheme, an ac/dc converter with power factor correction is connected to the line followed by a dc/dc converter. These two power stages can be controlled separately, and thus it makes both converters possible to be optimized. The drawbacks of this scheme is lower efficiency due to twice processing of the input power, larger control circuits, higher cost and low reliability.
A one-stage scheme combines the PFC circuit and power conversion circuit in one stage. Due to its simplified power stage and control circuit, this scheme is potentially more efficient. The underline strategy of this scheme is to design the circuit in a certain way that allows its PFC circuit and power conversion circuit to share the same power switch. Several PFC circuits have been reported. See for example: C. Cansem and L Barbi. xe2x80x9cA Unity of Power Factor Multiple Isolated Outputs Switching Mode Power Supply Using A Single Switch.xe2x80x9d APEC""91. pp. 430-436. These circuits are especially attractive in low cost, low power applications. However, some drawbacks still exist: a) owing to improperly sharing of the power switch, when the converter operates at high frequency, the unavoidable leakage inductance of their power transformers produce high voltage spike at the switching time resulting in decreased efficiency; b) because the power switch performs both PFC and regulation purposes, their regulation capabilities are limited; and, c) at high current and low duty ratio operation, a high voltage presents on the bulk capacitor, resulting in a high rating in design and hence raising the cost. Recently, several single switch converter topologies have been presented to overcome the above drawbacks. See for example: P. Kornetzky. H. Wei and I. Bartarseh. xe2x80x9cA Novel One-Stage Power Factor Correction Converter.xe2x80x9d IEEE APEC ""97 Proc. pp. 251-258; and, Y. S. Lee, IC W. Sui and B. T. Lin, xe2x80x9cSingle-Stage Isolated Power-Factor-Corrected Power Supplies with Regenerative Clamping.xe2x80x9d IEEE APEC ""97 Proc. pp. 259-265
U.S. Patents have been recently issued for AC/DC converters with power factor correction but fail to overcome all problems presented above. See for example. U.S. Pat. No. 5,224,025 to Divan et al; U.S. Pat. No. 5,416,387 and U.S. Pat. No. 5,442,539 to Oat et al; U.S. Pat. No. 5,479,331 to Lenni; U.S. Pat. No. 5,510,974 to Gu et al; U.S. Pat. No. 5,515,257 to Ishii: U.S. Pat. No. 5,559,688 to Pringle; U.S. Pat. No. 5,592,128 to Hwang; U.S. Pat. No. 5,594,629 to Steigerwald: U.S. Pat. No. 5,598,326 to Liu et al; U.S. Pat. No. 5,600,546 to Ho et al; and, U.S. Pat. No. 5,619,404 to Zak.
Our recently issued U.S. Pat. No. 5,959,849 dated Sep. 28, 1999 does overcome most of the problems by teaching an AC to DC converter which combines a boost circuit, a Pulse Width Modulation (PWM) switching regulator and a forward circuit power stage in which two storage capacitors are used to relieve the voltage spike produced by the power transformer and to provide energy to the output when the AC line voltage crosses zero. Unfortunately, the rapid evolution of computers and similar digital devices have required power supplies of voltages less than 5 volts and those that function in the hundreds of kilo-herz environments without severe energy losses and resultant failures.
The first objective of the present invention is to provide a switching power supply that operates from AC line voltage having a high power factor and output isolation.
The second objective of this invention is to provide a mechanism for soft-switching to allow for a one-stage power factor correction in an AC to DC converter operating in the hundreds of kilo-hertz.
The third objective of this invention is to provide an AC to DC converter having an output transformer that allows the converter to be used for single output and multi-output applications in the low voltage range.
The fourth objective of this invention is to provide an AC to DC converter where the leakage inductance of the forward mode power transformer will not cause an additional voltage stress at the power switch so that a power switch having a lower voltage rating and less power dissipation can be used.
The preferred embodiment of the AC to DC converters with PFC according to this invention is a power supply that provides a DC power to a load from an AC source comprising: a rectifying stage for transferring electrical energy from an AC source into pulsating unipolar voltage pulses at output terminals;
A boost stage having a controllable conducting means and a first unidirectional conduction means for controlling current flow from the output terminals of the rectifying state and blocking the current flow into the opposite direction and a single controllable switching device connected across the output terminals and more than one inductive-capacitive stages preferably two connected between said boost stage and said forward stage providing an inductive energy storing circuit when said switching device is closed and a capacitive charging circuit when said switching device is open, whereby a conversion efficiency of over approximately 75% AC to DC is achieved and with outlet voltage of approximately five volts and lower on alternatively a power on and off switch and an on and off auxiliary switch for controlling both the PFC circuit means and the AC to DC power conversion circuit means, wherein the converter operates at a frequency of greater than about 500 kilo-hertz.
Further objects and advantages of this invention will be apparent from the following detailed description of a presently preferred embodiment, which is illustrated, schematically in the accompanying drawings.